Ventricular assist devices (VAD) are receiving a lot of attention as a cure for heart failure. The cure for serious heart failure is possible by a ventricular assist device or a heart transplant. VAD is used as a bridge to the heart transplant. Furthermore, the use of VAD as destination therapy is expected. Therefore, VAD is a promising device as future heart failure treatment. However, VAD has a lot of problem with practical use, such as anti-thrombosis, anti-hemolysis, durability and safety. And, expansion of the application range by the further downsizing is demanded. In this study, the semi-open impeller for the micro centrifugal pump with 24mm impeller diameter was adopted. In the present paper, its performance and internal flow condition are investigated based on the numerical analysis results.